Will upvote all and any
5. why might a diverse population be more stable? (hint: think about how a diverse ecosystem may be more resistant to to desease, predetation, or invasion.)
7. what do you think might happen to the animals in an area if the plant diversity declines significantly? explain your reasoning.

5) This is a very broad question with several answers. But, a diverse population maybe more stable because there would be many species and organisms, which means that there will be less chance of inbreeding occurring. Inbreeding causes several genetic diseases in organism that may cause a drastic die off in the population. Also, A diverse population means that the food web in complex, but there is also a vast number of sources of food and this would alter the prey-predator interactions. 

7) If plant diversity declines, there would be a decline in the entire population... be it primary consumers, secondary consumers, tertiary consumes, or detrivores etc. The loss of the plant population means that you lose your primary food producers and then the herbivores will not have enough to eat and then there will not be enough prey for carnivores/predators. The whole food web would collapse which is why conservation is very important today! 

Hope that helped!

Yes yes yes very good